Probing vacuum birefringence in an Ultrastrong Laser Field via High-energy Gamma-ray Polarimetry

TL;DR

This paper proposes a novel, compact method to detect vacuum birefringence using high-energy gamma-ray polarimetry in an ultrastrong laser field, potentially enabling the first laboratory observation of this quantum electrodynamics effect.

Contribution

It introduces a self-probing scheme combining electron beams and petawatt lasers to measure vacuum birefringence without complex synchronization, supported by nonperturbative QED simulations.

Findings

Clear vacuum birefringence signature detected in simulations

Conversion of circular to linear polarization observed

High-contrast asymmetry in pair distributions predicted

Abstract

Vacuum birefringence (VB), a fundamental prediction of nonlinear quantum electrodynamics (QED), has eluded direct laboratory detection due to its extreme weakness. We propose a compact, "self-probing" scheme where a GeV electron beam collides head-on with a petawatt laser pulse. Circularly polarized gamma-ray photons, generated via nonlinear Compton scattering in the same pulse, then probe the birefringent vacuum it induces. This integrated design bypasses the stringent synchronization and beam transport requirements of traditional pump-probe setups. Our nonperturbative strong-field QED simulations reveal a clear VB signature: conversion of circular to linear polarization, with the induced Stokes parameter $S_1$ reaching ~0.019 within the selected angular range. This corresponds to a refractive index difference $\Delta n = 1.829 \times 10^{-4}$ over micron-scale paths, directly measurable as a high-contrast "X-shape" asymmetry in $e^+e^-$ pair distributions. The scheme provides a feasible path to first laboratory VB detection with current laser and accelerator technologies.